Filtration, continuous actual area

External-Cake Tubular Filters Several filter designs are available with vertical tubes supported by a filtrate-chamber tube sheet in a vertical cylindrical vessel (Fig. 18-115). The tubes may be made of wire cloth porous ceramic, carbon, plastic, or metal or closely wound wire. The tubes may have a filter cloth on the outside. Frequently a filter-aid precoat will be applied to the tubes. The prefilt slurry is fed near the bottom of the vertical vessel. The filtrate passes from the outside to the inside of the tubes and into a filtrate chamber at the top or the bottom of the vessel. The sohds form a cake on the outside ofthe tubes with the filter area actually increasing as the cake builds up, partially compensating for the increased flow resistance of the thicker cake. The filtration cycle continues until the differential pressure reaches a specified level, or until about 25 mm (1 in) of cake thickness is obtainea... 

The rotary drum filter is a continuous filtration device, because it does not have to be shut down during the cycle, although the operation is still cyclic. A schematic is shown in Fig. 13-8. The drum rotates at a rate N (rpm), and the filter area is the total drum surface, i.e., A = izDL. However, if the fraction of the drum that is in contact with the slurry is /, then the length of time in the cycle during which any one point on the surface is actually filtering is f/N ... 

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